The present invention relates to a traveling wave electro-optic modulator based on an organic electro-optic crystal
Electro-optic modulators are useful for modulating light signals with radio frequency or higher frequency signals. Typically, electro-optic modulators are used to modulate signals onto laser light beams for use in fiber optic communications systems. It is well known to fabricate electro-optic modulators from crystals composed of LiNbO3 and similar substances. Such modulators are typically constructed by ion bombardment or dopant diffusion into LiNbO3 waveguides.
However, such prior art modulators are subject to problems. For example, the modulation sensitivity of LiNbO3 based modulators is limited by the electro-optic coefficient of the LiNbO3 material itself. One known solution to this limitation is to construct electro-optic modulator from organic polymers rather than LiNbO3. However, organic polymers also have modulation sensitivity limitations due to the density of chromophores in the organic polymer material and the alignment efficiency of the chromophores by a poling process during modulator fabrication. In addition, exposure of such materials to environmental extremes may adversely affect the performance of electro-optic modulators constructed of such materials.
A need arises for an electro-optic modulator that provides improved modulation sensitivity and improved environmental characteristics.
The present invention is an electro-optic modulator, a system including an electro-optic modulator, and a method for producing an electro-optic modulator. The electro-optic modulator of the present invention provides improved modulation sensitivity and improved environmental characteristics.
An electro-optic modulator, according to the present invention, comprises: a substrate having a surface, a first optical waveguide and a second optical waveguide, the optical waveguides formed in the substrate and being co-planar, each waveguide having a first index of refraction, each waveguide operable to transmit a light signal, and the substrate having a second index of refraction, a first electrode disposed on the surface of the substrate between the first and second optical waveguides, the first electrode operable to receive a modulation signal, and a second electrode and a third electrode disposed on the surface of the substrate surrounding the first and second optical waveguides, the second and third electrodes connected to a common potential, whereby the light signal is modulated in accordance with the modulation signal.
The waveguides may be formed by any of several well-known processes, such as dopant diffusion, etching, or photobleaching. These processes typically work by lowering the index of refraction of the substrate from its original value, while leaving the index of refraction of the waveguides unchanged. The waveguides are masked during the process, which prevents exposure to the index lowering chemicals or radiation.
The second index of refraction may be lower than the first index of refraction. The optical waveguides may be formed by changing an index of refraction of the substrate from the first index of refraction to the second index of refraction by photobleaching of the substrate. The optical waveguides may be substantially aligned lengthwise with a crystalline axis of the substrate. The light signal may be a laser light signal. The substrate may be formed from a crystalline material. The substrate may be formed from diethylaminosulfur trifluoride.
An electro-optic modulator system, according to the present invention, comprises: a light source operable to output a light signal, a modulation signal generator operable to output a modulation signal, and an electro-optic modulator comprising: a substrate having a surface, a first optical waveguide and a second optical waveguide, the optical waveguides formed in the substrate and being co-planar, each waveguide having a first index of refraction, each waveguide coupled to the light signal, and the substrate having a second index of refraction, a first electrode disposed on the surface of the substrate between the first and second optical waveguides, the first electrode coupled to the modulation signal, and a second electrode and a third electrode disposed on the surface of the substrate surrounding the first and second optical waveguides, the second and third electrodes connected to a common potential, whereby the light signal is modulated in accordance with the modulation signal.
The light source may comprise a laser device. The system may further comprise an optical splitter coupled to the laser device, the optical splitter operable to output a first light signal and a second light signal, the light signals being similar. The first optical waveguide may be coupled to the first light signal and the second optical waveguide may be coupled to the second light signal. The system may further comprise an optical combiner having a first input coupled to a first modulated light signal output from the first optical waveguide, a second input coupled to a second modulated light signal output from the second optical waveguide, and an output operable to output a combined modulated light signal.
The second index of refraction may be lower than the first index of refraction. The optical waveguides may be formed by changing an index of refraction of the substrate from the first index of refraction to the second index of refraction by photobleaching of the substrate. The optical waveguides may be substantially aligned lengthwise with a crystalline axis of the substrate. The substrate may be formed from a crystalline material. The substrate may be formed from diethylaminosulfur trifluoride.
A method of producing an electro-optical modulator, according to the present invention, comprises the steps of: applying a first mask and a second mask to a substrate, the substrate having a first index of refraction, exposing the substrate and masks to light at a first angle to a perpendicular from the substrate, whereby a portion of the substrate that is not shielded by the masks is photobleached by the light so as to change an index of refraction of the portion of the substrate that is not shielded by the masks, and exposing the substrate and masks to light at a second angle to a perpendicular from the substrate, the second angle of similar magnitude to the first angle and of opposite direction to the first angle, whereby a portion of the substrate that is not shielded by the masks is photobleached by the light so as to change an index of refraction of the portion of the substrate that is not shielded by the masks.
The method may further comprise the steps of: removing the first and second masks, applying a first electrode to a surface of the substrate between the first and second optical waveguides, and applying a second electrode and a third electrode to the surface of the substrate surrounding the first and second optical waveguides. The optical waveguides may be substantially aligned lengthwise with a crystalline axis of he substrate. The substrate may be formed from a crystalline material. The substrate ay be formed from diethylaminosulfur trifluoride.